The invention relates generally to a photomask and, more particularly, to a method for fabricating a photomask using a self-assembled molecule layer.
A photomask functions to form a desired pattern on a wafer while light is irradiated on a mask pattern formed on a substrate and selectively transmitted light is transferred onto the wafer. As the photomask, a binary mask made by forming a light shielding pattern containing chromium (Cr) on a transparent substrate and having a light transmitting region for transmitting light therethrough and a light shielding region for shielding the light is generally used. However, as the degree of integration of semiconductor devices increases and thus pattern sizes are reduced, it has become difficult to precisely form a desired pattern due to diffraction or interference of light transmitted through the binary mask. To overcome the difficulty in precisely forming the pattern, a phase shift mask using a phase shift material having a transmittance of several percent has been developed and used.
In the phase shift mask, a mask pattern to be transferred onto a wafer is formed by forming a phase shift layer and a light shielding layer on a transparent substrate and implementing two patterning processes including writing and pattern transfer steps. A first patterning process is implemented to form the mask pattern to be transferred onto the wafer, primarily to a light shielding layer. The light shielding pattern formed in the first patterning process functions as a mask for etching to form a phase shift pattern thereunder. A second patterning process is implemented to selectively remove the light shielding pattern. By removing the light shielding pattern on the circuit pattern with the second patterning process, an intended phase shift pattern is exposed.
Next, the pattern is transferred onto the wafer using the photomask, and a critical dimension of the pattern formed on the wafer is measured to determine whether it is within tolerance, to thereby determine whether the photomask must be re-fabricated. Meanwhile, the critical dimension of the pattern is measured using a scanning electron microscope (SEM) of a type designed for critical pattern measurement. The scanning electron microscope measures the critical dimension by supplying electrons onto a pattern extracted as a sample and detecting reflected secondary electrons.
However, when measuring the critical dimension with the scanning electron microscope, there is a problem that charges accumulate on the transparent substrate, which is a non-conductor, and thus an image defect is generated. It is difficult to treat such a scanning electron microscope for conductivity with, e.g., platinum (Pt). To improve the problem of charge accumulation, a method of grounding to a surface of the light shielding layer of the photomask is employed. However, as the pattern of the photomask becomes finer, neutralization of the charges accumulated on the transparent substrate becomes more difficult. When the charges induced, resolution is lowered or a deviated landing is caused upon the measurement with the scanning electron microscope, resulting in a measuring error. Accordingly, there is a problem that it becomes difficult to measure the precise critical dimension.